The S-SAFE sampling is expected to end around 3 p.m. on Tuesday. The dates of the other two testing days will be announced the day before.

The purpose of the study is to determine the best course of action in the event chemical, biological or radiological materials are released in the city, whether intentionally or not. The research will help officials decide on evacuation or sheltering plans.

In addition to the air sampling, researchers will also release low concentrations of harmless gases known as perfluorocarbons at several subway and street-level locations in Manhattan for 30 minutes within the first few hours of the study.

The NYPD said perfluorocarbon tracer gases do not pose any health or environmental hazard. They are non-toxic, inert, odorless and invisible, and have been used in airflow studies since the 1980s, including a 2005 Urban Dispersion Program conducted in Manhattan.

S-SAFE was commissioned by the NYPD and funded through a $3.4 million Department of Homeland Security Transit Security Grant. It is the first of its scale to study air flow in a dense, complex urban environment both below and above ground, according to officials.

“The NYPD works for the best but plans for the worst when it comes to potentially catastrophic attacks, such as ones employing radiological contaminants or weaponized anthrax,” Police Commissioner Ray Kelly said back when the study was announced in April. “This field study with Brookhaven’s outstanding expertise will help prepare and safeguard the city’s population in the event of an actual attack.”

“Brookhaven Lab is a world leader in the use of tracer gases to study airflow, and we are excited about this opportunity to apply that expertise to enhancing the safety of New York City residents and emergency responders,” Brookhaven Lab Director Doon Gibbs said in April.

The two other testing days will be held on non-consecutive days in July, barring poor weather conditions.